Photoconductive detector device with plasmonic electrodes

ABSTRACT

A photoconductive device that includes a semiconductor substrate, an antenna assembly, and a photoconductive assembly with one or more plasmonic contact electrodes. The photoconductive assembly can be provided with plasmonic contact electrodes that are arranged on the semiconductor substrate in a manner that improves the quantum efficiency of the photoconductive device by plasmonically enhancing the pump absorption into the photo-absorbing regions of semiconductor substrate. In one exemplary embodiment, the photoconductive device is arranged as a photoconductive source and is pumped at telecom pump wavelengths (e.g., 1.0-1.6 μm) and produces milliwatt-range power levels in the terahertz (THz) frequency range.

STATEMENT OF FEDERALLY-SPONSORED RESEARCH

This invention was made with government support under N66001-10-1-4027awarded by Navy/SPAWAR, W911NF-12-1-0253 awarded by the Army ResearchOffice, N00014-11-1-0856 awarded by the Office of Naval Research,EECCS1054454 Awarded by the National Science Foundation,N00014-11-1-0096 awarded by the Office of Naval Research. The governmenthas certain rights in the invention.

TECHNICAL FIELD

This invention relates generally to photoconductive devices and, moreparticularly, to photoconductive devices that emit or detect terahertz(THz) waves using plasmonic electrodes for improved quantum efficiency.

BACKGROUND OF THE INVENTION

Terahertz (THz) waves possess a number of unique capabilities andproperties, including ones that make them useful for chemicalidentification, material characterization, biological sensing, andmedical imaging, to cite a few examples. Terahertz Time DomainSpectroscopy (THz-TDS) is a spectroscopic technique that uses very shortpulses of THz radiation to probe or analyze different properties of amaterial and is sensitive to the material's effect on both the amplitudeand phase of the THz radiation. Although there is much potential for thecommercial use of THz-TDS systems, their use thus far has been somewhathindered by certain drawbacks, such as their low power, inefficiency,high cost, thermal breakdown, complexity, and the bulky nature ofexisting terahertz sources.

For example, most existing terahertz (THz) spectrometers are not broadlyused for military and commercial chemical detection and/orcharacterization purposes. This is mainly due to the drawbacks mentionedabove which can hinder the practical feasibility of such systems,particularly in portable systems. Some research has been conducted inthe areas of frequency domain terahertz spectrometers utilizing coherentterahertz sources, solid-state terahertz sources, quantum-cascade lasers(QCLs), and nonlinear optical techniques for down-conversion toterahertz frequencies, to name a few, however, each of these approacheshas drawbacks of its own. Finding an approach that offers suitableoutput power and efficiency across a wide range of terahertz (THz) ornearby frequencies, yet does so in a relatively compact form and undernormal operating conditions, can be challenging.

SUMMARY OF THE INVENTION

According to one aspect, there is provided a photoconductive device foremitting or detecting terahertz (THz) radiation. The photoconductivedevice may comprise: a semiconductor substrate; an antenna assemblyfabricated on the semiconductor substrate; and a photoconductor assemblyfabricated on the semiconductor substrate and coupled to the antennaassembly, the photoconductor assembly includes one or more plasmoniccontact electrodes. The photoconductive device receives optical inputfrom at least one optical source and uses the plasmonic contactelectrodes to improve the quantum efficiency of the photoconductivedevice.

According to another aspect, there is provided a method of operating aphotoconductive device that has a semiconductor substrate, an antennaassembly, and one or more plasmonic contact electrodes. The method maycomprise the steps of: (a) receiving optical input from an opticalsource at the semiconductor substrate; (b) promoting the excitation ofsurface plasmon waves or surface waves with the plasmonic contactelectrodes, wherein the surface plasmon waves or surface waves influencethe optical input from the optical source so that a greater amount ofoptical input is absorbed by the semiconductor substrate and results inphotocurrent in the semiconductor substrate; (c) applying a voltage tothe antenna assembly so that a first portion of the photocurrent in thesemiconductor substrate drifts toward a first antenna terminal and asecond portion of the photocurrent in the semiconductor substrate driftstoward a second antenna terminal; and (d) emitting terahertz (THz)radiation from the photoconductive device in response to the first andsecond antenna terminals being supplied with the first and secondportions of photocurrent.

According to another aspect, there is provided a method of operating aphotoconductive device that has a semiconductor substrate, an antennaassembly, and one or more plasmonic contact electrodes. The method maycomprise the steps of: (a) receiving optical input from an opticalsource at the semiconductor substrate; (b) promoting the excitation ofsurface plasmon waves or surface waves with the plasmonic contactelectrodes, wherein the surface plasmon waves or surface waves influencethe optical input from the optical source so that a greater amount ofoptical input is absorbed by the semiconductor substrate and results inphotocurrent in the semiconductor substrate; (c) receiving incidentterahertz radiation through the antenna assembly which induces aterahertz electric field across the plasmonic contact electrodes; and(d) drifting the photocurrent toward the plasmonic contact electrodes asa result of the induced terahertz electric field which generates anoutput photocurrent that is proportional to a magnitude of the incidentterahertz radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments will hereinafter be described inconjunction with the appended drawings, wherein like designations denotelike elements, and wherein:

FIG. 1 shows an exemplary photoconductive device, where the particularphotoconductive device shown here is arranged as a photoconductivesource with a dipole antenna and has an enlarged section showing severalplasmonic contact electrodes in more detail;

FIGS. 2-3 b are different graphs showing certain properties of exemplarysemiconductor substrate materials;

FIG. 4 shows another exemplary photoconductive device, where theparticular photoconductive device shown here is arranged as aphotoconductive source with a bowtie antenna and has an enlarged sectionshowing several plasmonic contact electrodes in more detail;

FIG. 5 shows exemplary antenna and photoconductor assemblies that may beused with a photoconductive device, such as the one in FIG. 1, and hasenlarged sections showing plasmonic contact electrode arrays in moredetail;

FIG. 6 shows another exemplary photoconductive device, where theparticular photoconductive device shown here is arranged as aphotoconductive source with a dipole antenna and has an enlarged sectionshowing several cross-shaped plasmonic contact electrodes in moredetail;

FIGS. 7a-c show exemplary plasmonic contact electrodes that may be usedwith a photoconductive device, such as the one in FIG. 1, where FIG. 7ashows two-dimensional plasmonic contact electrodes and FIGS. 7b-c showthree-dimensional plasmonic contact electrodes;

FIG. 8 shows a schematic cross-section of a portion of an exemplaryphotoconductive device, such as the one in FIG. 1, where the devicefurther includes a passivation layer on top of the semiconductorsubstrate;

FIGS. 9a -13 are different graphs and schematic illustrations showingvarious properties of exemplary photoconductive devices and theiroperation;

FIGS. 14-15 are illustrations of exemplary setups for characterizing aphotoconductive source, such as the one of FIG. 1, as a photoconductivesource or emitter; and

FIGS. 16a-d are different graphs comparing a conventionalphotoconductive terahertz source with an exemplary photoconductivesource, such as the one of FIG. 1, which has a number of nano-spacedplasmonic contact electrodes.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The photoconductive devices described herein address some of theperformance limitations of existing photoconductive devices. Accordingto an exemplary embodiment, the photoconductive device may be ahigh-performance plasmonic-distributed photoconductive terahertz sourcethat is pumped at telecom pump wavelengths (e.g., 1.0-1.6 μm) for whichhigh power, narrow linewidth, frequency tunable, compact andcost-efficient optical sources are available. A photoconductor assemblyhaving a number of distributed plasmonic contact electrodes maysignificantly enhance the pump coupling efficiency, while enabling ultrahigh-speed collection of photo-generated carriers and mitigating thecarrier screening effect and thermal breakdown limitations that canoccur while operating at high pump power levels. This, in turn, mayenable terahertz power levels that are significantly higher than thosecurrently available. According to one exemplary implementation, thephotoconductive device is a frequency tunable, compact and light-weightterahertz radiation source (pulsed or non-pulsed) that receives opticalinput from two commercially available laser diodes and an erbium-dopedfiber amplifier that is capable of producing milliwatt-range powerlevels in the 1 THz frequency range. Such a high power and compactterahertz source could be beneficial for a number of potentialapplications, including next generation material, chemical and/orbiological sensors. Although the following description is directed to aphotoconductive source that emits terahertz radiation, it should beappreciated that the teachings are equally applicable to aphotoconductive detector that receives terahertz radiation instead ofemitting it. Thus, the term “photoconductive device,” as used herein,includes both photoconductive sources and photoconductive detectors, andis not limited to one or the other.

The unique capabilities of material, chemical and/or biological sensorsbased on terahertz (THz) spectrometry offer an exceptional platform forstandoff detection of many concealed toxic chemicals and gases,explosives, pathogens, chemical and biological agents. This is becausemany individual chemicals have their distinctive thermal emission peaksor their rotational or vibrational emission lines in the terahertzportion of the electromagnetic spectrum (e.g., 0.1-10 THz). Apart frompotential security and military applications, terahertz spectrometersoffer very promising platforms for environmental and space studies,biological analysis, pharmaceutical and industrial quality control, tocite a few possibilities. It should be appreciated that while thephotoconductive device described herein is directed to use withterahertz (THz) radiation, it is not limited to such and may be usedwith electromagnetic radiation outside of the THz range (e.g.,frequencies significantly higher than 10 THz and wavelengthssignificantly smaller than 1550 nm).

Some of the most powerful continuous wave (CW) terahertz sources to datehave been molecular gas lasers, p-germanium lasers and free-electronlasers, which provide enough output power for most spectrometry systems,but can be bulky and expensive and generally are not suitable forportable systems. Additionally, electron beam devices such asbackward-wave oscillators and travelling wave tube (TWT) regenerativeamplifiers can offer reasonably high output power levels atsub-millimeter wave frequencies, but have not demonstrated efficientoperation above about 1.2 THz and are not easily tunable. Some otherconventional terahertz sources include IMPATT diodes, backward waveoscillators, Gunn diodes, frequency multipliers, and MMICs, while somemore recent terahertz sources include photomixers, resonance tunnelingdiodes, traveling wave tubes, quantum cascade lasers, and sources basedon nonlinear optical effects, to name a few.

Turning now to FIG. 1, there is shown an exemplary embodiment of aphotoconductive device 10 that is arranged as a photoconductive sourceor emitter and includes a semiconductor substrate 12, an antennaassembly 14, a photoconductor assembly 16, and a lens 18. Somepotential, non-limiting features or attributes of photoconductive device10 include: a photoconductor assembly 16 that uses plasmonic contactelectrodes to enhance the quantum efficiency of the terahertzphotoconductive device; a photoconductor assembly 16 that uses adistributed architecture to suppress thermal breakdown limitations andthe carrier screening effect; antenna and photoconductor assemblies 14,16 that enable a directional, high-intensity and steerable terahertzoutput; a photoconductive device 10 that is frequency tunable, compactand lightweight and is capable of producing radiation with an outputpower of at least 1 mW in a frequency range of 0.1-10 THz; and asemiconductor substrate 12 that includes a thin layer or film withrelatively few defects acting as an efficient photo-conductingsemiconductor at telecom wavelengths where high-power, tunable, narrowlinewidth and compact lasers are available. Other features andattributes are certainly possible, as the aforementioned represent onlysome of the possibilities. Those skilled in the art will appreciate thata photomixer is a photoconductive device that is used to mix two opticalbeams; if the same device is illuminated with a single beam it would notgenerally be a photomixer. Photoconductive device 10 may be used as aphotomixer (two beams) or as a pulsed terahertz source (one beam), tocite two possibilities.

As mentioned before, one of the main quantum efficiency limitations ofexisting photoconductors is the low thermal conductivity of thesemiconductor substrates, which can lead to premature thermal breakdownat high optical pump power levels. Low thermal conductivity can becomemore problematic in some high defect materials where the photonmean-free-path is dramatically reduced by the introduced defects.Photoconductive device 10 aims to mitigate the quantum efficiencylimitations of certain conventional devices that are based on shortcarrier lifetime semiconductors by using a photoconductor assembly 16with plasmonic contact electrodes that are distributed on asemiconductor substrate 12 having a low defect layer or film. Theplasmonic contact electrodes may suppress the carrier screening effectand thermal effects by spreading high-power optical input across anarbitrarily large two-dimensional array of electrodes that are adjacentthe various elements of antenna assembly 14; this, in turn, distributesthe capacitive load across the antenna and photoconductor assemblies 14and 16. The plasmonic contact electrodes may also be configured in aperiodic arrangement with sub-wavelength spacing (e.g., 100 nm spacingfor a pump wavelength of 1 μm) in order to maintain high quantumefficiency and ultrafast response by enhancing the opticaltransmissivity into the semiconductor substrate and absorption in closeproximity with the plasmonic contact electrodes.

Semiconductor Substrate—

Semiconductor substrate 12 acts as a photoconductive material that cangenerate electron-hole pairs in response to optical input from one ormore optical sources, such as an ultrafast pulsed lasers or twoheterodyning continuous-wave lasers used in the telecom industry.According to an exemplary embodiment, semiconductor substrate 12includes a thin top layer or film 30 (e.g., a thin layer around 1 μmthick) that acts as the device active layer, and a thicker bottom layer32 that acts as a base layer on which the top layer can be grown ordeposited. In one potential embodiment, thin top layer 30 is made fromgermanium (Ge) and thicker bottom layer 32 is made from silicon (Si); inanother potential embodiment, thin top layer 30 is made from indiumgallium arsenide (InGaAs) or gallium arsenide (GaAs) and thicker bottomlayer 32 is made from indium phosphate (InP). Other materials andsemiconductor substrate arrangements and layer combinations arecertainly possible, including any suitable combination or use ofsapphire; silicon (Si); germanium (Ge); silicon germanium (SiGe);various indium gallium arsenide compounds (InGaAs) including those thatare crystalline, low-temperature-grown, ion-implanted and erbiumarsenide doped; various gallium arsenide compounds (GaAs) includingthose that are crystalline, low-temperature-grown, ion-implanted, erbiumarsenide doped; various indium gallium nitride compounds (InGaN); indiumphosphide (InP); and Graphene, to cite a few of the possibilities.

It should be mentioned that the photoconductive device with plasmoniccontact electrodes disclosed herein provides for high quantum efficiencyand ultrafast operation simultaneously, and thus enables the use of manysemiconductor materials that could not be used in previousphotoconductive devices (e.g., germanium (Ge), Graphene, and indiumgallium nitride compounds (InGaN)). One potential advantage of germanium(Ge) is the higher thermal conductivity it offers compared with othermaterials, while offering high absorption coefficients attelecommunication optical wavelengths. As illustrated in FIG. 2, bulk-Geand some of the InGaAs-based compounds (e.g., In0.53Ga0.47As) offer muchhigher absorption coefficients at standard telecom wavelengths, comparedto other semiconductors which are traditionally used forphoto-absorption. Additionally, it has been shown that adding strain toGe films can push the absorption band edge to even longer wavelengthsand increase the absorption coefficient significantly at the standardtelecom wavelength range (˜1.55 μm). A potential advantage of usingindium gallium nitride compounds (InGaN) relates to power handlingadvantages for device operation at high power levels. Also, Graphene mayoffer extremely fast carrier transport speeds, further enhancing thedevice quantum efficiency while operating at ultrafast speeds. It shouldbe recognized, however, that the photoconductive device 10 describedherein may use any number of suitable semiconductor materials, includinglow-defect or high-defect materials, and is not limited to any of theexemplary materials provided above.

Addressing the thermal limitations which can limit the maximum outputpower of a photoconductive device can be helpful for developing asuitable, high-power terahertz source. In the absence of such thermallimitations, photoconductor terahertz output power scales quadraticallywith the pump power level, which shows great potential for achievingvery high terahertz power levels. According to previous studies, thethermal conductivity of bulk Ge and In_(0.53)Ga_(0.47)As are measured tobe about 0.58 W cm⁻¹° C.⁻¹ and 0.05 W cm⁻¹° C.⁻¹, respectively. Althoughnot necessary, it is preferable that the thin top layer 30 of thesemiconductor substrate 12 have a thermal conductivity that is equal toor greater than 0.1 W cm⁻¹° C.⁻¹. The introduction of defects mightaffect the thermal conductivity of the grown crystalline film,particularly introduced defects that appear at the multi-layer interfaceand can potentially reduce the thermal conductivity by reducing phononmean free-path. Accordingly, it is preferable that thin top layer 30 bea low-doped, low-defect layer with as high of thermal conductivity aspossible (this is not absolutely necessary, however, as the thin toplayer may sometimes be comprised of high-defect, short carrier lifetimematerials instead).

Another potentially attractive feature of a thin top layer 30 made fromgermanium (Ge) is its compatibility with a thicker bottom layer 32 madefrom silicon (Si) layer, as well as process compatibility with silicon(Si) processes (e.g., low temperature processing capability). In someinstances, this may be a significant capability because it allowsgrowing thin layers of germanium (Ge) active areas on a high resistivitysilicon (Si) substrate, which is known to introduce minimal propagationloss at terahertz wavelengths. The exemplary semiconductor substrateembodiment with its germanium (Ge) and silicon (Si) layers may takeadvantage of existing epitaxial growth techniques for growing highquality Ge layers on Si.

The exemplary semiconductor substrate 12 can have a thin layer of Gefilm that is grown on high resistivity Si. One potential technique foraccomplishing this includes multiple steps of growth and annealing in ahydrogen ambient to grow high quality germanium (Ge) on silicon (Si)with low threading dislocation density, as is known in the art. In thistechnique, a thin Ge film is grown heteroepitaxially on Si and in-situannealed at a higher temperature in an H₂ ambient which can reduce thesurface roughness by 90% and facilitates stress relief in the first fewhundred angstroms. Subsequent Ge growth is homoepitaxy on a virtual Gelattice with no additional defects forming, where the grown film takesthe crystal lattice of the underlying substrate. A potential advantageof the employed Ge growth technique is that the resulting strained-Gefilm can offer an order of magnitude higher absorption coefficient attelecom wavelengths (e.g., about 1550 nm wavelength), compared to bulkGe. This is due to a germanium (Ge) absorption spectrum red shift(illustrated in FIG. 3b ), as a result of tensile strain introduced bythe thermal expansion coefficient mismatch between Si and Ge (shown inFIG. 3a ). FIG. 3b illustrates Ge absorption spectrum red shift due totensile strain and, more specifically, experimental absorptioncoefficient vs. photon energy of a Ge layer grown with an estimatedtensile strain of 0.16%, showing a 47 nm red shift of the absorptionedge due to tensile strain. FIG. 3a shows the change in the bands withbiaxial tensile strain in the Ge film. For more information on thegraphs illustrated in FIGS. 3a-b , please refer to A. K. Okyay, A.Nayfeh, T. Yonehara, A. Marshall, P. C. McIntyre, K. C. Saraswat, “HighEfficiency MSM Photodetectors on Heteroepitaxially Grown Ge on Si,” Opt.Lett., 31, pp. 2565-2567, 2006. The minimum of the conduction band atthe zone center moves down while the maximum of the valence band forheavy and light holes move up. The direct band energy is reduced due totensile strain. In the case of an indium gallium arsenide (InGaAs)compound, the thin layer can be epitaxially grown on an Indium aluminumArsenide (InAlAs) buffer layer on a semi-insulation Indium Phosphide(InP) substrate. Some of the above-listed fabrication techniques areknown and understood by those skilled in the art.

Antenna Assembly—

Antenna assembly 14 is fabricated on semiconductor substrate 12 and isused to emit terahertz (THz) radiation (photoconductive source) or todetect THz radiation (photoconductive detector). Those skilled in theart will appreciate that any number of different photoconductive THzantenna configurations and arrangements may be used with thephotoconductive device described herein and that it is not limited toany one particular embodiment. For example, antenna assembly 14 may be amonopole antenna, butterfly antenna, a dipole antenna, a spiral-typeantenna, a folded dipole antenna, a log-periodic antenna, a bow tie-typeantenna, or any other suitable photoconductive THz antenna configurationthat is known in the art. In FIG. 1, antenna assembly 14 isschematically shown as a dipole antenna, while FIG. 4 schematicallyshows a different embodiment of the photoconductive device 10 using abow tie-type antenna assembly 34, instead of a dipole-type antenna. Inboth of these embodiments, the antenna assemblies use a number ofplasmonic contact electrodes to improve the performance of the devices,as described below in further detail.

In order to implement photoconductive device 10 as a photoconductiveterahertz (THz) source, the photoconductor assembly 16 is integratedwith a THz antenna. Since the amplitude of the photo-generated currentfed to the antenna is linearly proportional to the photoconductor activearea, an array of closely spaced dipole antennas may be designed toincrease the photoconductive area while maintaining a small RCtime-constant and high radiation resistance. ADS and HFSS softwarepackages can be used to optimize, for example, an antenna arraystructure for maximum radiation power, by combining the antennaradiation parameters, photoconductor parasitics, and the amount ofinjected current based on photoconductor active area.

The output power of the photoconductive device 10 can be furtherenhanced through use of resonant cavities, antennas with higherradiation resistance and bandwidth, antenna arrays, and through the useof appropriate impedance matching techniques. For example, any of thetechniques, features, embodiments, etc. disclosed in C. W. Berry, M.Jarrahi, “Principals of Impedance Matching in Photoconductive Antennas,”Journal of Infrared, Millimeter and Terahertz Waves, 33, 1182-1189,2012, the entire contents of which are incorporated herein by reference,may be used with the exemplary photoconductive device disclosed herein.

Photoconductor Assembly—

Photoconductor assembly 16 is also fabricated on semiconductor substrate12 and is designed to improve the quantum efficiency of thephotoconductive device 10 by plasmonically enhancing the pump absorptioninto the photo-absorbing regions (e.g., layer 30) of semiconductorsubstrate 12. According to an exemplary embodiment, photoconductorassembly 16 includes a number of plasmonic contact electrodes 50 thatare arranged in arrays in order to enhance the optical-to-terahertzconversion efficiency in the photoconductive source 10. “Plasmoniccontact electrode,” as used herein, broadly refers to any electrodestructure that is part of or is coupled to a photoconductor assembly andexcites or otherwise promotes surface plasmon waves or surface waveswhich enhance light coupling to sub-wavelength device active regionsdetermined by the electrodes, as described below in more detail. In oneembodiment, the plasmonic electrodes 50 include a number of thinfinger-like electrodes that are arranged in parallel and are fabricatedon the top layer 30 of the semiconductor substrate 12 so thatsub-wavelength metallic apertures are formed there between. Plasmonicenhancement is achieved by configuring the closely spaced plasmoniccontact electrodes 50 in a periodic arrangement with nano-scaleelectrode width and electrode spacing (e.g., less than about 200 nm),which allows ultrafast collection of the photo-generated carriers andresults in higher quantum efficiency compared with previousphotoconductors. The plasmonic contact electrodes 50 may be integrallyformed with the antenna assembly 14 such that they extend therefrom, orthey may be a separate array or other structure that is formed separateand is then connected to the antenna assembly. Some non-limitingexamples of suitable plasmonic contact electrode metals include gold(Au), silver (Ag), titanium (Ti), nickel (Ni), and various alloysthereof.

Skilled artisans will appreciate that optical absorption intosub-wavelength contact electrode apertures or gaps is typically severelylimited by the diffraction limit. However, photoconductor assembly 16with its periodic arrangement of plasmonic contact electrodes 50circumvents the diffraction limit by configuring sub-wavelength contactelectrodes in a way that is capable of exciting surface plasmon waves.Excitation of surface plasmon waves allows bending of the electric fieldlines of the incident optical beam on top of the periodically arrangedplasmonic contact electrodes 50, which in turn prevents the incidentoptical excitation from being blocked by electrodes 50. This can be ofsome significance, especially for certain embodiments where asignificant portion of the device active area is covered by plasmoniccontact electrodes 50. Plasmonic electrodes with metallic apertures orslits may be implemented in broadband terahertz spatial beam modulators,and plasmonically-enhanced localization of light into photoconductiveantennas which enhance the efficiency/bandwidth product ofphotoconductive antennas, are also possibilities.

As explained more in the following sections, incorporating plasmoniccontact electrodes 50 in a photoconductor assembly 16 can overcomecertain diffraction limitations, which can significantly reduce thetransmission of an optical beam through contact electrodes withsub-wavelength spacing. As a result, ultrafast transport ofphoto-generated carriers will not pose a significant limitation onquantum efficiency. One of the challenges of device designs with anarrow band-gap semiconductor, such as Ge, is the high dark current. Thehigh dark current challenge can be addressed by using asymmetricplasmonic contact electrodes, considering the plasmonic contact metal,feature size, and shape.

A schematic representation of excited surface plasmon waves is shown inthe enlarged inset of FIG. 4. The electrode grating geometry is designedto excite surface plasmon waves along the periodic metallic gratinginterface upon incidence of a TM-polarized optical pump. Excitation ofsurface plasmon waves allows transmission of a large portion of theoptical pump through the nanoscale grating into the photo-absorbingsemiconductor substrate 12. It also enhances the intensity of theoptical pump in very close proximity to the plasmonic contact electrodes50. As a result, the average photo-generated electron transport pathlength to the anode electrode or terminal of the antenna assembly 14 issignificantly reduced in comparison with conventional photoconductiveemitters. Therefore, it is desirable for the photoconductor assembly 16to optimize the optical pump transmission into the photo-absorbingsemiconductor substrate 12 while minimizing the electrode spacing tominimize the average photo-generated electron transport path length tothe anode electrode.

With reference now to FIG. 5, there are shown antenna and photoconductorassemblies 14 and 16 where several enlarged insets illustrate plasmoniccontact electrode arrays in more detail. As demonstrated, approximatelyhalf of the plasmonic contact electrodes 50 are electrically coupled toa first antenna terminal 24 and provide that terminal withphoto-generated carriers, while the other half of the plasmonic contactelectrodes 50 are electrically coupled to and provide a second antennaterminal 26 with photo-generated carriers. An elongated separatingmember 28, which is an optional component, separates or partitions thetwo groups of plasmonic electrodes 50. One half of the plasmonic contactelectrodes is collecting positive carriers and the other half iscollecting negative carriers. Although the spacing and configuration mayvary from that shown in FIG. 5 by way of example, the spacing betweenthe plasmonic contact electrodes can be of some importance. If, forexample, the plasmonic contact electrodes that are connected to antennaterminal 24 are too close to those connected to antenna terminal 26,there could be capacitance-related issues or it could prevent thehigh-enough electric field levels required for ultrafast drift of thephotocarriers to the plasmonic contact electrodes along the entireplasmonic contact electrode area. Furthermore, even though thephotoconductor assembly 16 is shown to be partitioned or arranged insegments or clusters of plasmonic contact electrodes with spacing inbetween, this is not necessary.

FIG. 6 shows another potential embodiment of the photoconductive source,with a somewhat different photoconductor assembly design.Photoconductive device 110 includes a semiconductor substrate 112,antenna assembly 114, photoconductor assembly 116, and a lens 118 and issimilar to the embodiment illustrated in FIG. 1, except that thephotoconductor assembly includes a number of cross-shaped plasmoniccontact electrodes 130. Photoconductor assembly 116, as well as theother photoconductor assemblies disclosed herein, may function as aplasmonic aperture and can be a two-dimensional array of nano-scaleapertures of various geometries and configurations, such as those havingthe following shapes: a grating, a rectangular-shape, a cross-shape, aC-shape, a H-shape, a split-ring-resonator, a circular hole, or arectangular hole (the ones shown in FIG. 6 being cross-shaped).

The use of high aspect ratio plasmonic contact electrodes embeddedinside the photo-absorbing semiconductor allows a larger number ofcarriers generated in close proximity with photoconductor contactelectrodes and, thus, enables further terahertz radiation enhancement.In this regard, extending the plasmonic electrode height to dimensionslarger than the optical pump absorption depth allows ultrafast transportof the majority of photocarriers to the photoconductor contactelectrodes and their efficient contribution to terahertz generation.This may eliminate the need for using short carrier lifetimesemiconductors, which may be used for suppressing the DC current ofphotoconductive emitters and for preventing undesired destructiveinterferences in continuous-wave photoconductive emitters. Eliminatingthe need for using short carrier lifetime semiconductors, which havelower carrier mobilities and thermal conductivities compared to highquality crystalline semiconductors, could have an important impact onfuture high power and high efficiency photoconductive terahertzemitters. It could also lead to a new generation of photoconductiveterahertz emitters based on photo-absorbing semiconductors with uniquefunctionalities (e.g., Graphene-based photoconductive emitters thatbenefit from superior carrier mobilities or GaN-based photoconductiveemitters that benefit from superior thermal conductivity).

FIG. 7a shows a photoconductor assembly with a number of two-dimensionalplasmonic contact electrodes 50, as described above, while FIGS. 7b-cshow a photoconductor assembly having a number of three-dimensionalplasmonic electrodes 150. “Two-dimensional plasmonic contactelectrodes,” as used herein, generally refer to plasmonic electrodesthat are very thin, thus, they have a small height dimension relative totheir width or length dimensions (see example schematically representedin FIG. 7a ). “Three-dimensional plasmonic contact electrodes,” as usedherein, generally refer to plasmonic electrodes that are much thickerand have a larger height dimension when compared to their width orlength dimensions (see example schematically represented in FIG. 7b ).Three-dimensional plasmonic contact electrodes extend away from thesurface of the semiconductor substrate by a more significant amount thantwo-dimensional electrodes and are sometimes described as having a highaspect ratio. These plasmonic contact electrode arrangements haveresulted in one or more orders of magnitude of terahertz radiationenhancement compared with conventional designs. Some estimates were maderegarding the impulse response and responsivity of the analyzedphotoconductors with the illustrated two- and three-dimensionalplasmonic contact electrodes by combining the photogenerated carrierdensity in a photo-absorbing substrate, the electric field data, and theclassical drift-diffusion model in a multi-physics finite-element solver(COMSOL) and calculating the induced photocurrent in response to anoptical pump impulse. These estimates indicate more than one order ofmagnitude terahertz power enhancement for a photoconductive THz emitterwith three-dimensional plasmonic contact electrodes (FIGS. 7b-c ) incomparison with a similar photoconductive device having two-dimensionalplasmonic contact electrodes (FIG. 7a ). Therefore, a photoconductiveterahertz emitter with the three-dimensional plasmonic contactelectrodes 150 may offer more than three orders of magnitude higherterahertz radiation compared with a conventional photoconductiveterahertz emitter lacking plasmonic contact electrodes, when comparedunder the same operation conditions.

Other photoconductor assembly and plasmonic contact electrodeembodiments may also be used with the photoconductive device 10disclosed herein, as the preceding examples are simply meant toillustrate some of the possibilities.

Other Components, Arrangements, Features, Etc.—

It is possible for photoconductive device 10 to include a variety ofother optical and non-optical components in order to improve theperformance, operation, etc. of the device. For example, in order toachieve higher quantum efficiencies and terahertz powers, specificallydesigned optical diffusers, one-dimensional or two-dimensional lensarrays can be used to guide the optical input or pump beam so that it isonly incident on the active areas of the photoconductor assembly 16 andis not wasted on the rest of the device area that does not contribute toTHz radiation, while employing large arrays of photoconductive devices.

Another potential feature that may be used is a dielectric passivationlayer that can reduce the Fresnel reflection at the semiconductorinterface and, thus, enhance optical pump transmission into thephoto-absorbing semiconductor substrate. While optical pump transmissioninto the photo-absorbing semiconductor of a conventional photoconductoris the result of direct interaction between the pump wave and thesemiconductor interface, optical pump transmission into thephoto-absorbing semiconductor of a photoconductor having plasmonicelectrodes, such as the one described herein, involves coupling to theexcited surface plasmon waves. In the case of a plasmonicphotoconductor, passivation layer thickness can be optimized to achieveup to 100% optical transmission into the photo-absorbing semiconductor.One non-limiting example of a suitable passivation layer 160 is shown inFIG. 8, where a SiO₂ passivation layer (e.g., 150 nm thick) is used withthe present photoconductor having plasmonic contact electrodes. It ispossible for the passivation layer 160 to completely encapsulate theplasmonic electrodes 50 (as shown in the drawing), or for thepassivation layer to just cover the top surface of the semiconductorsubstrate 12 so that the plasmonic contact electrodes 50 extend throughthe passivation layer. The passivation layer 160 may include number ofsuitable materials other than SiO₂, such as SiN or Si₃N₄, for example.

A low resistivity bias network may also be used in conjunction with theproposed photoconductive device 10. Such a bias network can beespecially useful when using low bandgap energy semiconductors (e.g. Ge,InGaAs), which have a relatively high dark current.

As mentioned above, the photoconductive device 10 may also be used as aphotoconductive terahertz detector as well. Similar to photoconductiveterahertz emitters, a primary limitation of conventional photoconductiveterahertz detectors is their low responsivity and sensitivity, which istypically the result of the inherent tradeoff between high quantumefficiency and ultrafast operation of conventional photoconductors.Another potential advantage of the plasmonic contact electrodestructures described herein is that they offer significantly higherresponsivities and detection sensitivities compared to conventionalphotoconductive terahertz detectors by reducing the photo-generatedcarrier transport path to the photoconductor contact electrodes.Moreover, the device active area can be increased without a significantincrease in the photoconductor capacitive parasitic and, therefore,higher detector responsivity levels can be achieved at higher opticalpump power levels. Performance of photoconductive terahertz detectorprototypes is characterized in a time-domain terahertz spectroscopysetup. Some experimental results show that incorporating plasmoniccontact electrodes may enhance the detector responsivity by more thanone order of magnitude.

Operation—

Operation of the photoconductive source 10 is based on an incidentoptical pump generating electron-hole pairs in the photo-absorbingsemiconductor substrate 12. An applied voltage across the photoconductorassembly 16 drifts the generated carriers toward their correspondingplasmonic contact electrodes 50. The collected photo-current at theplasmonic contact electrodes 50 drives the terahertz antenna assembly14, which is connected to or otherwise integrated with thephotoconductor assembly 16. The generated photo-current follows thewaveform of the optical pump, thus, by using a sub-picosecond opticalpulse or heterodyning two continuous-wave optical beams with a terahertzfrequency difference, a pulsed or continuous-wave terahertz current iscoupled to the terahertz antenna assembly 14, respectively. In order tooperate efficiently at the desired terahertz frequency range, thetransport time of the photo-generated carriers to the plasmonic contactelectrodes 50 should be a fraction of the terahertz oscillation period.

The photoconductive source 10 may exhibit a sub-picosecond response timeand can be fabricated on a high quality crystalline semiconductorsubstrate 12 with a large carrier lifetime or a short-carrier lifetimesemiconductor substrate. The ultrafast response of the photoconductivesource 10 is due to excitation of surface plasmon waves whichconcentrate a major portion of the incident light in close proximitywith the plasmonic contact electrodes 50 of the photoconductor assembly16 and, thus, enables ultrafast collection of photo-generated carrierswithout sacrificing the photoconductor quantum efficiency significantly.In one embodiment, the ultrafast photoconductive source 10 includes aphoto-absorbing semiconductor substrate 12 made from a high-qualitycrystalline semiconductor and a photoconductor assembly 16 having aplasmonic contact electrode grating. The grating periodicity or pitchand the electrode spacing or aperture size are significantly smallerthan the wavelength of the incident optical beam. Therefore, surfaceplasmon waves can be excited at the metallic grating surface. Excitationof surface plasmon waves assists with efficient transmission of theincident light, through the sub-wavelength grating apertures, into thephoto-absorbing semiconductor substrate 12. Additionally, the intensityof the transmitted optical wave is enhanced in close proximity with theplasmonic contact electrodes 50, thereby reducing the averagephoto-generated carrier transport time to the plasmonic contactelectrodes and enabling high quantum efficiency and ultrafast operationsimultaneously.

Since electrons have significantly higher mobilities compared to holesand due to the nonlinear increase in the bias electric field nearcontact electrodes, the optical pump or optical source may be focusedonto the photoconductive gap of the photoconductor assembly 16asymmetrically close to the anode contact of the antenna assembly 14 tomaximize terahertz radiation. Put differently, the photoconductivedevice 10 described herein may be asymmetrically pumped where an opticalsource pumps just one of the contact electrodes of the antenna assembly14, or it may be symmetrically pumped where the optical source pumpsboth of the contact electrodes of the antenna.

In summary, photoconductive source 10 includes a photoconductor assembly16 with nano-scale plasmonic contact electrodes 50 that significantlyreduce the photo-generated carrier transport path and enable ultrafastoperation without the need for short-carrier lifetime substrates whichmay limit the efficiency of conventional photoconductive terahertzsources. The ability to achieve ultrafast operation while maintaininghigh quantum efficiency may be valuable for future high-power terahertzemitters. The following paragraphs discuss other potential aspects ofthe photoconductive device 10 and its operation.

Manufacturing—

The following description is of a manufacturing or fabrication processand is directed to a certain non-limiting embodiment wherethree-dimensional plasmonic contact electrodes and a gallium arsenide(GaAs) semiconductor substrate are used. As already mentioned numeroustimes, the photoconductive device is not so limited and may include anynumber of the various features and embodiments described herein. Thisexemplary fabrication process may start with depositing a SiO₂ filmfollowed by patterning a nanoscale metal grating (e.g., Ni) to form ahard mask for etching nanoscale GaAs gratings. The SiO₂ film and theunderlying GaAs substrate are etched afterwards according to the metalhard mask. Plasmonic contact electrodes may then be formed by sputteringTi/Au followed by liftoff. Finally, a dielectric (e.g., SiO₂)passivation layer can be deposited to cover the top of GaAs gratings.

Use of focused ion-beam to pattern thicker photoresists layers with highaspect ratios is also possible. For a large number of photoconductorarrays, nano-imprinting, self-assembly techniques, and a focusedion-beam may be employed to achieve better uniformity and fasterpattering.

Skilled artisans will appreciate that any number of other manufacturingor fabrication processes may be used instead.

Tests, Simulations, Findings, Setups, Etc.—

The following paragraphs, and the figures that they reference, describedifferent tests, simulations, findings, setups, etc. for exemplaryembodiments of photoconductive source 10 and are meant to illustratevarious operational aspects of that device. The specific embodimentsutilized in these tests and simulations are not meant to be limiting,and are simply provided to further explain or illustrate differentfeatures or aspects of the present photoconductive device.

The impact of plasmonic contact electrodes 50 in enhancing the inducedphotocurrent in ultrafast photoconductors and the radiated terahertzpower from photoconductive terahertz emitters is significant. Theenhancement concept can be similarly applied to enhance the radiationpower from photoconductive terahertz emitters as well as the detectionsensitivity of photoconductive terahertz detectors with a variety ofterahertz antennas with and without interdigitated contact electrodes,as well as large-area photoconductive devices in both pulsed andcontinuous-wave operation. Put differently, the various plasmoniccontact electrodes disclosed herein may be used with a wide variety ofother components, including different semiconductor substrates, antennaassemblies, photoconductor assemblies, lenses, bias networks, etc.

One potential advantage of the proposed photoconductive device 10 is itsscalability, which can allow plasmonically enhanced optical transmissioninto an arbitrarily large device area. This is because the devicecapacitive parasitic is distributed along the radiating antenna length,so neither degrades antenna efficiency nor antenna impedance matching asa function of frequency. Similar plasmonic electrode configurations maybe used to enhance the quantum efficiency of ultrafast distributedphotoconductors with closely spaced contact electrodes.

Yet another potential advantage of the proposed photoconductive device10 is the enhancement of photo-generated carrier concentration near theplasmonic contact electrode regions and the resulting improvement inquantum efficiency.

Another aspect of the proposed photoconductive device 10 involves theability to suppress the carrier screening effect. The screening effectstarts to be effective at high pump intensities, when a large number ofphoto-generated carriers, accumulated in a small area, screen thecarrier drifting electric field. To suppress the carrier screeningeffect, the maximum pump intensity is determined at which the screeningelectric field starts degrading the photoconductor terahertz current byreducing the carrier drift velocity. The overall dimensions of thephotoconductor can be chosen such that at a given pump power, the pumpintensity does not exceed this limit. The pump coupling efficiency,pump/terahertz wave propagation loss, together with the pump/terahertzwave velocity matching requirement have been substantial obstacles toprevious photoconductive devices that limit their maximum terahertzoutput power. Photoconductive source 10 does not face the samelimitations associated with traveling-wave distributed photoconductorssince the process of pump coupling into the photoconductor and terahertzdecoupling out of the photoconductor are usually performed in free spaceand in parallel, as understood by those skilled in the art.

In order to suppress the carrier screening effect and thermal effects,photoconductive source 10 spreads the high power optical pump on arelatively large photoconductor active area consisting oftwo-dimensional arrays of plasmonic contact electrodes or elements. Inconventional designs, the capacitive loading from a large photoconductoractive area can severely limit the photo-generated terahertz currentcoupled to the radiating antenna because of the RC roll-off. To solvethis limitation, photoconductive source 10 integrates photoconductorassembly 16 as a distributed capacitive load along antenna assembly 14.

Yet another potential advantage of photoconductive device 10, comparedto other THz sources, is the continuous and broad frequency tunabilityof the device. The frequency tuning range of photoconductive source 10with its plasmonic contact electrodes 50, like most otherphotoconductive THz sources, will be determined by the radiationbandwidth of the employed THz antenna.

According to another potential aspect of the photoconductive device 10,the broad distribution of the optical pump on the two-dimensionalplasmonic arrays may help achieve a highly directive output terahertzbeam. A highly directional radiation pattern of the proposedphotoconductive device 10 can be desirable, especially for terahertzremote sensing and standoff chemical detection applications, where theuse of terahertz collimating/focusing components may be restricted.Additionally, output terahertz radiation from the photoconductive device10 can be controllably deflected by specific distribution of biasvoltage applied to photoconductor elements, V_(bias) ^((i)).

Another desirable aspect of the proposed photoconductive device 10 isits high-efficiency operation at or around the 1065 nm and 1550 nmwavelength. In the popular fiber-optic telecommunication band around1550 nm and 1065 nm, compact erbium-doped and Ytterbium-doped fiberamplifiers have been developed that can boost the power ofspectrally-pure laser-diode sources such as external-cavity diode lasers(ECDL) up to tens of watts. While the high tunability of fiber lasers(e.g., about 10 s of nm) allows continuous broadband (e.g., several THz)spectral measurements, their narrow spectral linewidth (e.g., less than1 MHz) enables accurate identification of chemicals with closely spacedabsorption lines (e.g., about 10 s of MHz spacing). Additionally,optical fibers are an ideal environment for combining two frequencyoffset lasers while consuming a very small volume and a minimalsensitivity to vibrations. For the proposed photoconductorcharacterization, two commercially available erbium fiber laser (ELTSeries) from IPG Photonics may be used as an optical source. This laseroffers a wavelength tuning range of 1540-1605 nm, a spectral linewidthof less than 300 KHz, wavelength stability of less than 0.1 nm over 30minutes, and an output power of up to 25 W with power stability of 0.1dB. Other optical sources may be used instead, as the preceding is onlyan example.

Referring back to FIG. 8, there is shown interaction of a TM-polarizedoptical beam (λ=1550 nm) with a photoconductor with a gold contactgrating (pitch=120 nm, metal width=60 nm) on a thin In_(0.53)Ga_(0.47)Assubstrate, calculated by a finite-element method solver COMSOL. Thearrows (representing the optical power flow direction) show how thepropagating light bends on top of the metallic grating to allow highefficiency coupling to the photo-absorbing substrate. As shown in FIG.8, the optical beam is intensely localized near the corners of themetallic grating, resulting in a very high concentration ofphoto-generated electron-hole pairs in the immediate vicinity of thephotoconductor contact electrodes. As displayed by FIG. 9a , the opticaltransmission spectrum of the grating is broadband at wavelengths whereIn_(0.53)Ga_(0.47)As is absorbing, and the InP substrate is transparent.At a wavelength of 1550 nm more than 5% of the incident optical power isabsorbed in the 60 nm thick In_(0.53)Ga_(0.47)As layer. The rest of theoptical power passes through the transparent substrate without leadingto additional heating. FIG. 9a illustrates optical transmission andexternal quantum efficiency through a gold grating (pitch=120 nm, metalwidth=60 nm, height=20 nm) into a 60 nm thick In_(0.53)Ga_(0.47)As layeron an InP substrate.

With reference to FIG. 9b , the photocurrent impulse response of theanalyzed photoconductor can be calculated by using the optical intensityprofile in the substrate, with the assumption that the photocarrierstravel along the bias electric field lines, and under a bias voltage of0.4 V. As a baseline for comparison, the photocurrent impulse responseof a conventional photoconductor has been calculated with a contactgrating pitch of 2 μm and duty cycle of 0.05 fabricated on an infinitelythick, photo-absorbing In_(0.53)Ga_(0.47)As substrate with a carrierlifetime of 300 fs. The photocurrent impulse responses are normalized bythe photoconductor active area and are illustrated in FIG. 9b . Comparedwith the conventional photoconductor, a photoconductor source 10 withplasmonic contact electrodes 50 may offer an order of magnitude higherphotocurrent impulse response. Therefore, integration of photoconductorassembly 16 having a number of plasmonic contact electrodes 50 with theantenna assembly 14 may provide significantly higher terahertz powersthan existing photoconductive terahertz sources. Additionally, utilizingcertain semiconductor substrate materials, such as In_(0.53)Ga_(0.47)As,can make photoconductive source 10 compatible with widely availabletelecommunication fiber lasers and components. Therefore,photoconductive source 10 is very promising for practicalphotoconductive terahertz generation, enabling ultrafast and highquantum efficiency operation, simultaneously.

FIGS. 10a-b demonstrate a similar plasmonic enhancement of optical pumptransmissivity into a semiconductor substrate that includes a thin toplayer 30 made of germanium (Ge), as opposed to InGaAs. Morespecifically, FIG. 10a illustrates how the transmission of aTM-polarized optical excitation into a germanium (Ge) layer is affectedby the plasmonic contact electrode 50 geometry at various opticalwavelengths. For a metal thickness of 20 nm, more than 50%transmissivity can be expected at 1550 nm wavelength by using less than100 nm plasmonic contact electrode width and spacing. The maximumtransmissivity is limited by the reflection at the semiconductor-airinterface. FIG. 10b , on the other hand, shows a cross-sectional opticalpower density distribution along a periodic arrangement of plasmoniccontact electrodes 50 with 60 nm electrode width and spacing at a 1550nm pump wavelength. This figure illustrates how the TM-polarized opticalfield lines are bent on top of the metallic electrodes to achieve a hightransmissivity into germanium (Ge) layer 30. It is possible for aplasmonic electrode configuration to offer up to 100% transmissivity byusing very thick metal electrodes.

Turning now to FIG. 11, there is shown a graph that illustrates how aphotoconductor active area can be scaled as a function of the opticalpump power to prevent photoconductor quantum efficiency degradationcaused by junction heating. In order to prevent device output powerdegradation at high pump power levels, semiconductor substrate 12 may bedesigned so that the junction temperature of the thin top layer 30, suchas one made from germanium (Ge), does not exceed the maximum junctiontemperature (e.g., T_(J max)=200° C.), above which rapid degradation anddevice failure can occur. This may allow the maximum optical pump powerto be defined as a function of the exposed device area. According tosome previous studies on photoconductor thermal heating, the Jouleheating, P_(Q), and the junction temperature, T_(J), can be approximatedby the following formulas:

T _(j) =T ₀ +P _(Q) /k√(2πA)  (1)

P _(Q)=(P _(opt1) +P _(opt2))/2*(1+RV _(B))≈(P _(opt1) +P_(opt2))/2*(1+RE _(BW))  (2)

where T₀ is the temperature of the surrounding environment, k is thebulk thermal conductivity, A is the photoconductor active area, P_(opt1)and P_(opt2) represent the incident optical power from the two frequencyoffset lasers, R represents the photoconductor responsivity (measured tobe 0.65 for one of our exemplary embodiments), V_(B) is the bias voltageacross the contact or plasmonic contact electrodes 50 which isapproximately E_(BW) at the maximum photoconductor quantum efficiency,E_(B) is the breakdown field in Ge, and w is the contact or plasmoniccontact electrode 50 spacing in the photoconductor. FIG. 11 shows howthe photoconductor active area can be scaled as a function of theoptical pump power to prevent photoconductor quantum efficiencydegradation caused by junction heating; the junction temperature is keptless than 200° C. in this example. Compared to high-defect InGaAs basedphotoconductors (demonstrated with plot 60), Ge-based photoconductors(demonstrated with plot 62) may allow an order of magnitude higheroptical pump powers for the same distributed active area without anydegradation in device quantum efficiency. This is attributed to thesignificantly lower thermal conductivity in certain high-defectsemiconductor compounds.

FIGS. 12a-b summarize the process of photo-generated terahertz currentcoupling into the terahertz radiating dipole antenna assembly and theexpected output terahertz power. To address the quantum efficiencylimitation of conventional photoconductive terahertz sources based onshort-carrier lifetime semiconductors, photoconductive source 10utilizes nanoscale photoconductor contact electrode gratings tosimultaneously achieve both ultrafast and high quantum efficiencyoperation. This photoconductive terahertz source circumvents the need ofa short-carrier lifetime semiconductor substrate, allowing for use ofhigh quality, crystalline substrates. FIG. 12a is a schematic diagramthat illustrates operation of a conventional photoconductive terahertzemitter based on short-carrier lifetime photo-absorbing semiconductors,and FIG. 12b illustrates an embodiment of photoconductive source 10 thatis based on nanoscale contact electrode gratings on a high-qualitycrystalline substrate.

FIG. 12b shows the schematic diagram and operation of an exemplaryembodiment of photoconductive source 10 based on nanoscale contactelectrode gratings. The photoconductor contact electrodes consist of twoarrays of nanoscale metallic gratings connected to the input port of adipole terahertz antenna. The grating periodicity (200 nm) and metalspacing (100 nm) is chosen to be smaller than the wavelength of theincident optical pump. Therefore, the grating geometry can bespecifically designed to allow efficient optical transmission throughthe metallic gratings into the photo-absorbing active region byexcitation of surface waves along the periodic metallic gratinginterface. Compared to conventional photoconductive terahertz sourceswith contact electrode spacing of ˜2 μm, the nanoscale spacing of thecontact electrode gratings significantly reduces the photo-generatedcarrier transport path to the photoconductor contact electrodes.Additionally, the 2 μm separation of the two nanoscale metallic gratingarrays maintains low capacitive loading to the terahertz antenna.Moreover, due to the excitation of surface waves along the periodicmetallic grating interface, the intensity of the transmitted opticalpump is significantly enhanced near the corners of the contactelectrodes, further reducing the average transport time of thephoto-generated carriers to the contact electrodes. Thus ultrafastphotoconductor operation can be achieved while maintaining a highquantum efficiency without the use of semiconductors with defect inducedshort-carrier lifetimes.

FIG. 12c shows the nanoscale grating designed for enhanced opticalabsorption for a TM-polarized 1550 nm optical pump. This opticalwavelength was chosen due to its compatibility with fiber-optictelecommunication wavelengths at which high power, wavelength tunable,and compact optical sources are commercially available. In order toachieve high optical absorption at 1550 nm pump wavelength,In_(0.53)Ga_(0.47)As is chosen as the photo-absorbing semiconductor.Using a gold grating with a 200 nm periodicity, 100 nm metal spacing,and 50 nm metal thickness, efficient transmission of more than 65% ofthe incident optical pump into the In_(0.53)Ga_(0.47)As photo-absorbingregion is achieved. The thickness of the photo-absorbingIn_(0.53)Ga_(0.47)As layer (60 nm) on the InP substrate is chosen tomaintain a sub-picosecond device response time and low DC photocurrentby preventing photocarrier generation at deep semiconductor regions thatdo not contribute to terahertz radiation generation. The photo-generatedcarrier concentration and optical power flow in the cross section of thedesigned photoconductor is shown in FIG. 12c . Since the excited surfacewaves exist at the metal-dielectric interface, regions near the cornersof the gold electrodes exhibit the largest generation of carriers. Theproximity of these carriers to the electrodes reduces the averagecarrier transport time.

FIG. 12d demonstrates that by combining the photo-generated carrierprofile with carrier transport dynamics under the influence of the biaselectric field, the impulse response current of the designedphotoconductor generated by an optical impulse is calculated andcompared with the impulse response current of a conventionalphotoconductor design with a contact grating pitch of 2 μm and metalwidth of 100 nm fabricated on an infinitely thick In_(0.53)Ga_(0.47)Assubstrate with a carrier lifetime of 0.3 ps. The results indicate thatthe photoconductor based on nanoscale contact electrode gratings (plot70) may offer an order of magnitude higher impulse response currentcompared with a conventional photoconductor (plot 72) for the samephotoconductor active area and optical pump intensity (FIG. 12d ). Plot70 projects the calculated impulse-response current of the designedphotoconductor (with 200 nm grating pitch, 100 nm width, and 50 nm thickAu gratings fabricated on a 60 nm thick In_(0.53)Ga_(0.47)As activelayer), and plot 72 projects that of a conventional photoconductor (witha contact grating pitch of 2 μm and metal width of 100 nm on aninfinitely thick In_(0.53)Ga_(0.47)As substrate with a carrier lifetimeof 0.3 ps).

The frequency tuning range of the exemplary photoconductive THz sourceis determined by the radiation bandwidth of the employed dipole antenna.It is possible to extend the operation bandwidth of the proposedphotoconductor by using plasmonic photoconductor array elements withdifferent dipole antenna lengths, or other antenna configurations withbroader radiation frequency (e.g., bow tie, spiral, and log-periodicantennas).

FIG. 13 illustrates the average terahertz output power of an exemplaryphotoconductive source 10 based on dipole antenna arrays as a functionof radiation frequency, and compares it with the terahertz output powerof a demonstrated photoconductive source, optimized for the maximumoutput power at a broad terahertz frequency range. Using twofrequency-offset optical pumps with 25 W pump power at 1.55 μmwavelength, a peak output terahertz radiation power of 15.5 mW at 1 THzand more than 1 mW output power is expected over the frequency range of(0.8 THz-1.5 THz). The expected output power (plot 80) is more thanthree orders of magnitude higher than previously demonstratedphotoconductive sources around 1 THz frequency range. FIG. 13 also showsthat the output terahertz power of the proposed photoconductor increasesquadratically as a function of optical pump power. At very high opticalpump powers, the photoconductor active area may be scaled to ensuresuppression of the thermal effects and the carrier screening effect andenable quadratic scaling of output terahertz power as a function ofoptical pump. In FIG. 13, the radiation peak is at the resonancefrequency which offers maximum antenna radiation resistance. Potentialoptical sources include commercially available optical sources for theoptical pump which are compact and portable.

FIG. 14 shows an exemplary setup for characterizing the photoconductivesource 10. Two frequency-offset lasers 90 are mixed in a fiber 92 via a3-dB fiber coupler. A fiber coupled collimator 94 is used to focus thebeam on to the photoconductive source 10. For pump powers beyond 10 W,free-space coupling may be used to prevent fiber melting. The outputterahertz beam is coupled to free space via a hyper-hemispherical Silens 18 with 5 mm radius and 1.76 mm setback, and sensed with apyroelectric detector. A chopper and lock in amplifier can be used tofilter out the low frequency noise of the pyroelectric detector. FIG. 15illustrates one possible embodiment of a next generation compact,cost-efficient, and high power tunable photoconductive source 100 basedon the plasmonic distributed photoconductor described herein.

FIGS. 16a-d compare a conventional photoconductive terahertz source withphotoconductive source 10 having a number of nano-spaced plasmoniccontact electrodes. In FIG. 16a , there is plotted measured terahertzradiation from the plasmonic and conventional terahertz emitters,electrically biased at 40 V, under various optical pump powers. Theinset curve shows the corresponding photocurrent. The error bars areassociated with the noise of a pyroelectric terahertz detector. FIG. 16bshows measured terahertz radiation versus collected photocurrent for theplasmonic and conventional terahertz emitters. The data represented inthe plot includes various bias voltages (10-40 V) under various opticalpump powers (5-25 mW). In FIG. 16c , relative terahertz powerenhancement, which is defined as the ratio of the terahertz poweremitted by the plasmonic terahertz emitter to the conventional terahertzemitter, is shown. Maximum enhancement is obtained at low optical powersbefore the onset of the carrier screening effect. And in FIG. 16d ,maximum terahertz power measured from the plasmonic and conventionalterahertz emitters under a 100 mW optical pump is illustrated. The biasvoltage of each device is increased until the point of device failure.

It is to be understood that the foregoing description is not adefinition of the invention, but is a description of one or morepreferred exemplary embodiments of the invention. The invention is notlimited to the particular embodiment(s) disclosed herein, but rather isdefined solely by the claims below. Furthermore, the statementscontained in the foregoing description relate to particular embodimentsand are not to be construed as limitations on the scope of the inventionor on the definition of terms used in the claims, except where a term orphrase is expressly defined above. Various other embodiments and variouschanges and modifications to the disclosed embodiment(s) will becomeapparent to those skilled in the art. All such embodiments, changes, andmodifications are intended to come within the scope of the appendedclaims.

As used in this specification and claims, the terms “for example,”“e.g.,” “for instance,” “such as,” and “like,” and the verbs“comprising,” “having,” “including,” and their other verb forms, whenused in conjunction with a listing of one or more components or otheritems, are each to be construed as open-ended, meaning that that thelisting is not to be considered as excluding other, additionalcomponents or items. Other terms are to be construed using theirbroadest reasonable meaning unless they are used in a context thatrequires a different interpretation.

1. A photoconductive device for detecting terahertz (THz) radiation,comprising: a semiconductor substrate; an antenna assembly fabricated onthe semiconductor substrate; and a photoconductor assembly fabricated onthe semiconductor substrate and coupled to the antenna assembly, whereinthe photoconductor assembly includes a plurality of plasmonic contactelectrodes and wherein the photoconductive device is configured toreceive optical input that impinges upon the photoconductor assembly andthe plasmonic contact electrodes such that charge carriers are generatedin the semiconductor substrate adjacent the plasmonic contact electrodesin response to the impinging optical input; wherein the plasmoniccontact electrodes have sub-wavelength electrode spacing compared to atleast one wavelength of the optical input, and wherein the plasmoniccontact electrodes are configured to collect the generated chargecarriers and are electrically connected to the antenna assembly, wherebythe plasmonic contact electrodes provide improved quantum efficiency ofthe photoconductive device when receiving the optical input.
 2. Thephotoconductive device of claim 1, wherein the semiconductor substrateincludes a first thin layer formed on a second thicker layer, and thephotoconductor assembly is fabricated on the first thin layer.
 3. Thephotoconductive device of claim 2, wherein the first thin layer of thesemiconductor substrate includes at least one material selected from thelist consisting of: sapphire, silicon (Si), germanium (Ge), silicongermanium (SiGe), indium gallium arsenide (InGaAs), gallium arsenide(GaAs), indium gallium nitride (InGaN), indium phosphide (InP), Grapheneor compounds thereof.
 4. The photoconductive device of claim 2, whereinthe first thin layer of the semiconductor substrate includes alow-defect thin film with a crystalline structure that is grown on thesecond thicker layer.
 5. The photoconductive device of claim 2, whereinthe first thin layer has a thermal conductivity that is equal to orgreater than 0.1 W cm⁻¹° C.⁻¹.
 6. The photoconductive device of claim 1,further comprising a dielectric passivation layer formed on thesemiconductor substrate, wherein the dielectric passivation layerencapsulates at least a portion of the plasmonic contact electrodes andenhances optical pump transmission into the semiconductor substrate. 7.The photoconductive device of claim 6, wherein the dielectricpassivation layer includes at least one material selected from the listconsisting of: SiN, Si₃N₄, or SiO₂.
 8. The photoconductive device ofclaim 1, wherein the antenna assembly includes at least one antenna typeselected from the list consisting of: a monopole antenna, butterflyantenna, a dipole antenna, a spiral-type antenna, a folded dipoleantenna, a log-periodic antenna, or a bow tie-type antenna.
 9. Thephotoconductive device of claim 1, wherein the plasmonic contactelectrodes have a sub-wavelength periodicity, electrode width andelectrode spacing that are all less than the wavelength of the opticalinput.
 10. The photoconductive device of claim 1, wherein the plasmoniccontact electrodes are arranged so that the intensity of the opticalinput is concentrated near the plasmonic contact electrodes such that ahigh concentration of photo-generated electron-hole pairs are located inthe immediate vicinity of the plasmonic contact electrodes.
 11. Thephotoconductive device of claim 1, wherein the plasmonic contactelectrodes are configured according to at least one shape selected fromthe list consisting of: a grating, a rectangular-shape, a cross-shape, aC-shape, a H-shape, a split-ring-resonator, a circular hole, or arectangular hole.
 12. The photoconductive device of claim 1, wherein theplasmonic contact electrodes are metallic electrodes and include atleast one metal selected from the list consisting of: gold (Au), silver(Ag), titanium (Ti), or nickel (Ni).
 13. The photoconductive device ofclaim 1, wherein the plasmonic contact electrodes are integrally formedwith the antenna assembly and extend from the antenna assembly.
 14. Thephotoconductive device of claim 1, wherein the plasmonic contactelectrodes are separately formed from the antenna assembly and areconnected to the antenna assembly.
 15. The photoconductive device ofclaim 1, wherein the plasmonic contact electrodes are two-dimensionalelectrodes that have a height dimension that is less than a width orlength dimension.
 16. The photoconductive device of claim 1, wherein theplasmonic contact electrodes are three-dimensional electrodes that havea height dimension that is greater than a width or length dimension. 17.A method of operating a photoconductive device having a semiconductorsubstrate, an antenna assembly, and one or more plasmonic contactelectrodes, comprising the steps of: (a) receiving optical input from anoptical source at the semiconductor substrate; (b) promoting theexcitation of surface plasmon waves or surface waves with the plasmoniccontact electrodes, wherein the surface plasmon waves or surface wavesinfluence the optical input from the optical source so that a greateramount of optical input is absorbed by the semiconductor substrate andresults in photocurrent in the semiconductor substrate; (c) receivingincident terahertz radiation through the antenna assembly which inducesa terahertz electric field across the plasmonic contact electrodes; and(d) drifting the photocurrent toward the plasmonic contact electrodes asa result of the induced terahertz electric field which generates anoutput photocurrent that is proportional to a magnitude of the incidentterahertz radiation.
 18. The photoconductive device of claim 1, whereinthe antenna assembly includes a first antenna terminal coupled to afirst lead of the photoconductive device and a second antenna terminalcoupled to a second lead of the photoconductive device, and wherein thephotoconductor assembly is at least partially located between the firstand second antenna terminals.
 19. The photoconductive device of claim18, wherein the photoconductor assembly includes a first plurality ofthe plasmonic contact electrodes electrically coupled to the firstantenna terminal and a second plurality of the plasmonic contactelectrodes electrically coupled to the second antenna terminal.
 20. Thephotoconductive device of claim 19, wherein each of the first and secondpluralities of plasmonic contact electrodes includes plasmonic contactelectrodes configured in a periodic array with parallel electrodes thathas sub-wavelength periodicity and excites surface plasmon waves.